1. Field of the Invention
This invention concerns a transmission unit and a method to drive a transmission unit.
2. Description of the Related Art
Transmissions which are equipped with both a mechanical transmission unit and a hydrodynamic transmission unit are well-known in numerous design variations. The hydrodynamic component of the transmission is typically designed to serve as a hydrodynamic coupling or hydrodynamic torque converter. These components, sometimes referred to as fluid drives, include an impeller and a turbine which together form at least one toroid-shaped operating zone. This space can be filled with an operating fluid, and this fluid is accelerated by the turbine and decelerated by the compressor. The resultant torque generation is essentially the result of inertia forces generated by the change in velocity of the fluid. The transmitted power is the product of the change in swirl (rotating momentum) of the fluid and the angular velocity. In addition to the compressor and turbine, a hydrodynamic torque converter also includes a "reactionary" component which is commonly referred to as stator. This stator is capable of absorbing torque. Since the sum of the moment forces in this circuitry has to equal zero, the torque of the turbine can, depending on the size and rotational direction of the stator torque, be larger, the same, or smaller than the torque induced by the compressor. In this manner, it is possible with the additional arrangement of only an impeller and a turbine, which is also referred to as hydrodynamic coupling, to substantially vary the rotational speed range and the torque of the turbine in relation to the impeller.
The transmitting of power hydrodynamically offers the advantages of being able to transmit significant power levels at high efficiency, low wear rates, and the use of transmission components that are comparatively small. Furthermore, these components are primarily used during run-up or during the initial acceleration phase of a vehicle that utilizes a power-generating device, as well as a power-consuming device. In transmission designs that are equipped with a hydrodynamic coupling as well as a gearbox, the hydrodynamic coupling serves the function of a "start-up" clutch. The torque or power transmission occurs consequently via the hydrodynamic unit only in first gear, usually during the initial acceleration phase. In other gears, the hydrodynamic unit is excluded from transmitting power. In addition, the transmission unit can include another fluid dynamic device such as a hydrodynamic retarder whose purpose it is to provide braking. This retarder can be an integral part of the transmission or can be an attachment to the transmission. The same task can also be performed by hydrodynamic torque converters which can work, in the lower gears, in a power-sharing mode together with the mechanical gears. Thereby, the hydrodynamic torque converter is usually used during the initial acceleration phase as a "start-up" or acceleration device. In the remaining gears, the power will be transmitted on a purely mechanical basis by bypassing the hydrodynamic converter. In the lower gears, the power will be transmitted via the hydrodynamic torque converter and the mechanical gearing of the transmission.
The present developments in the automobile industry are partially characterized by conflicting trends. In addition to the trends for more design space, more comfort and improved vehicle acceleration and deceleration behavior for the purpose of improved safety for all vehicles, the aspect of environmental protection increases in significance. One possibility of lowering CO.sub.2 emissions that are emitted by automobiles is the improvement of the energy conversion process (mechanical efficiency) of the drive train between the engine and the power consumer. Furthermore, the trend is to use smaller engines with smaller displacements which translates into lower torque and higher rated engine speeds. These are the requirements by which vehicle transmissions will be judged. The fundamental criterion in the assessment of the quality and character of the transmission are thereby determined by:
the driveability performance as determined by the performance range of the torque converter; PA1 the number of gears and gear ratios; and PA1 the fuel consumption as affected by the layout of the drive ratio in the highest gear as well as the efficiency of the transmission.
It is therefore desirable for transmissions to achieve optimum efficiency and torque converter range which, at a given gear selection, maximizes the use of the engine operating range. This, in essence, allows operation along the optimum fuel consumption curve. Above all, it is then possible to decrease fuel consumption, harmful exhaust emissions and aggregate noise emissions by increasing the gear ratio separation in suitable stages.
The automatic transmissions that are on the market today fulfill these stated requirements by and large very well. However, in order to effectively operate the internal combustion engine within the minimum fuel consumption operating area, in combination with lowest exhaust emissions, a sufficient number of gear ratios are required such that 5 and 6 speed transmissions are no longer rarities in the automobile industry. This has the consequence of an increase in the number of transmission related componentry. Additional building blocks are required for the achievement of additional functions or tasks such as the generation of braking torque, for example. As a rule, transmission configurations that include a hydrodynamic coupling or hydrodynamic torque converter, as well as a mechanical transmission unit, often also use a hydrodynamic retarder. In this case, solutions are to be devised to reduce windage losses during idling of the impeller since a retarder that has been fully emptied always develops a residual amount of torque as a result of frictional bearing losses and change in swirl (rotating momentum) of the air surrounding the components. The braking torque generated by this residual torque is very small. However, it could have a very negative effect at elevated rotational speeds and can lead to an unacceptable increase in the temperature of the retarder. To lower the losses associated with windage (aerodynamic blade pass losses), a series of solutions are already known. One of the solutions includes the use of stator bolts as well as the evacuation of the hydraulic circuit. These solutions, however, are very extravagant in their application and require additional design space and, therefore, a larger retarder. There are, however, known transmission configurations with hydrodynamic torque converters that use this device for the purpose of generating braking torque.
Based on the trends to engines with smaller displacements, which tend to require an ever smaller attachment diameter, it becomes increasingly difficult to apply hydrodynamic converters with small outline dimensions which are capable of transmitting the available engine torque at low rotational speeds. Specifically, drawbacks arise such as power transmission capability being acceptable only at elevated speeds. Since, however, the initial acceleration phase requires torque and not power, there are additional losses in power as a result of the elevated rotational speeds which are, in turn, reflected by an increase in fuel consumption. The associated increase in noise emissions places a burden on the environment. During braking of a vehicle with a typical drive line design, the internal combustion engine remains connected to the vehicle or to the output shaft of the transmission and, therefore, causes unnecessary noise emissions due to resultant high shaft speeds.
There exist many variations of transmission design. This is due to the required total gear ratio separation and the required shapes of the blading of the individual elements dictated by the numerous applications to internal combustion engines.